Newswise — When the song pauses in a game of musical chairs, everyone jostles for one of the remaining seats. Bird migration today is much the same. When it’s time for a break in their biannual travels, songbirds descend to rest and refuel, searching for respite in a dwindling number of forest patches.

Avian research often focuses on forests as breeding habitats, but scientists are working to understand the role that small forest patches play in migration — a vital portion of a bird’s lifecycle when you consider that some species spend as much as half the year in transit. Now, for the first time, a team of researchers from Princeton University and the University of Delaware has created a comprehensive map of migratory pathways and stopover locations in the Eastern United States.

“Small pockets of deciduous forest are often neglected in conservation planning because birds have low breeding success in these spaces,” said Princeton University doctoral candidate Fengyi Guo, lead author of the study. “But the entire population moves across the continent twice annually. Many of them depend on food and shelter in these forest pockets to complete their migration and a chain is only as strong as its weakest link. Successful conservation of migratory bird populations requires enough habitat to be protected at all stages of its annual cycle.”

Unlike waterfowl or shorebirds that are heavily dependent on wetlands and coastlines, scientists have not observed strong fidelity to specific stopover sites among songbirds. Migrating along a broad front across the continent, these forest-dwelling songbirds often stop wherever they can, making migration a perilous venture.

“Songbirds are often naive about the places that they’re stopping over; they don’t necessarily know where the resources are or where the dangers are,” said Jeff Buler, professor of wildlife ecology in UD’s College of Agriculture and Natural Resources. “When they’re in these unfamiliar places, they may be more susceptible to things like predation. Throughout the annual cycle, the migration period is a time when these birds experience the greatest mortality.”

Using weather radar data —  a technology prized for its utility in comprehensively tracking the nighttime movements of bird populations — the research team found that birds concentrate in certain regions and along certain pathways during migration. 

In the Eastern U.S., songbirds tend to loosely travel along the Mississippi River or along the Appalachian Mountains. Within these pathways, the study found that the large expanse of croplands in the mid-Western U.S. act as an ecological barrier, with birds congregating in high densities along the edges. This unexpected discovery is the first recorded evidence of a large man-made barrier to bird migration akin to natural barriers like large water bodies or deserts.

“The prairie biome is now dominated by croplands, especially along the Corn Belt that extends into Illinois, Indiana and Ohio. We found that this is acting as an ecological barrier similar to the way that the Sahara Desert in Africa is a barrier to European and African bird migration,” Buler said. “The Corn Belt is a habitat desert for these forest birds and they concentrate in forests along the southern edge of that biome before and after making the trek across.”

Researchers also detected high-density concentrations of birds in forest patches embedded within landscapes that otherwise have very little forest. That means that in developed suburban or urban areas, or in agricultural regions like the Corn Belt, even small patches of forest that dot the landscape provide refuge for significantly more birds than previously understood. 

“One way we think about it is that these little forest patches are wicking up all the birds in a larger landscape,” Buler said. “There are huge numbers of birds that are moving across the U.S. but when it comes to landing and finding stopover habitats, they have to search and settle for the closest habitat in these landscapes where there are sometimes very few options.” 

Underscoring the conservation value of these stopover sites, the study indicates that the protection of forested land is necessary, particularly in landscapes dominated by agriculture, to create a network of suitable habitats through which songbirds can travel. As many bird populations face rapid declines, stopover site preservation could help ease the burden of an otherwise vulnerable migration journey and help sustain healthy bird populations.

Journal Link: Proceedings of the National Academy of Sciences